/*
 * Copyright (c) 2014-2022 Monix Contributors.
 * See the project homepage at: https://monix.io
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package monix.reactive.internal.operators

import monix.execution.Ack.{ Continue, Stop }
import monix.execution.Scheduler
import monix.execution.cancelables.CompositeCancelable
import scala.util.control.NonFatal
import monix.execution.{ Ack, Cancelable }
import monix.reactive.Observable
import monix.reactive.observers.Subscriber

import scala.concurrent.Future

private[reactive] final class WithLatestFromObservable[A, B, +R](
  source: Observable[A],
  other: Observable[B],
  f: (A, B) => R
) extends Observable[R] {

  def unsafeSubscribeFn(out: Subscriber[R]): Cancelable = {
    val connection = CompositeCancelable()

    connection += source.unsafeSubscribeFn(new Subscriber[A] { self =>
      implicit val scheduler: Scheduler = out.scheduler

      private[this] var isDone = false
      private[this] var otherStarted = false
      private[this] var lastOther: B = _

      private[this] val otherConnection = {
        val ref = other.unsafeSubscribeFn(new Subscriber.Sync[B] {
          implicit val scheduler: Scheduler = out.scheduler

          def onNext(elem: B): Ack =
            self.synchronized {
              if (isDone) Stop
              else {
                if (!otherStarted) otherStarted = true
                lastOther = elem
                Continue
              }
            }

          def onComplete(): Unit = ()
          def onError(ex: Throwable): Unit =
            self.onError(ex)
        })

        connection += ref
        ref
      }

      def onNext(elem: A): Future[Ack] =
        self.synchronized {
          if (isDone) Stop
          else if (!otherStarted) Continue
          else {
            // Protects calls to user code from within the operator and
            // stream the error downstream if it happens, but if the
            // error happens because of calls to `onNext` or other
            // protocol calls, then the behavior should be undefined.
            var streamErrors = true
            try {
              val r = f(elem, lastOther)
              streamErrors = false
              out.onNext(r)
            } catch {
              case NonFatal(ex) if streamErrors =>
                self.onError(ex)
                Stop
            }
          }
        }

      def onError(ex: Throwable): Unit =
        signalComplete(ex)
      def onComplete(): Unit =
        signalComplete(null)

      private def signalComplete(ex: Throwable): Unit =
        self.synchronized {
          if (!isDone)
            try {
              isDone = true
              if (ex == null) out.onComplete()
              else out.onError(ex)
            } finally {
              otherConnection.cancel()
            }
        }
    })
  }
}
